Apparatus and process for threshing tobacco

ABSTRACT

An apparatus for threshing tobacco includes a rotatable stripper and a feeder which includes a pair of cooperating counter-rotatable elements for delivering tobacco leaves to the stripper. The feeder and the stripper are arranged such that, in use, the tobacco leaves experience shearing forces as they pass from between the counter-rotating elements to the rotating stripper. These forces at least partially strip the lamina from the stems.

This invention relates to apparatus and a process for threshing tobacco.

In the tobacco industry, it is well-known that in order to process thetobacco into a suitable form for use in the manufacturing of products,the tobacco leaf has to have the midrib stem removed from the rest ofthe tobacco leaf. This separation is sometimes done by hand but is morecommonly done using a threshing machine.

Threshing machines for tobacco, now in use, differ little from thatdescribed by Du Brul in U.S. Pat. No. 209,801 of Nov. 12, 1878, designedfor threshing tobacco for use as cigar filler.

In the current art, the most common leaf threshing process consists of:

1. Feeding the tobacco leaves into the top of a threshing mill. Theleaves are broken up by the action of a rotating element, with radiallyprotruding teeth, until they are small enough to pass through a fixedscreen at the bottom of the mill.

2. The threshed leaf is then classified using an air flow in a verticaltower. The lighter, stem free, material rises with the air flow and isremoved from the threshing process. The heavier, stem containing,material drops, under the influence of gravity, down the tower, throughthe air.

3. This heavy, stem containing material is passed to a second threshingmill, and the process is repeated.

The overall threshing process usually has between 4 and 6 stages ofthreshing and classifying before all the lamina is removed from themidrib stem.

The most common form of-threshing mill for tobacco is described by Allenin U.S. Pat. No. 2,760,492 and Bonner et al in U.S. Pat. No. 3,141,485.

U.S. Pat. No. 2,962,029 (McCashen) describes a tobacco threshing machinehaving a single rotating threshing element mounted inside a rotatingdrum.

Various documents describe machines which include two or more rotatingthreshing elements inside a single machine with various claims forimprovements over existing single rotor threshers. Bonner et al in U.S.Pat. Nos. 3,126,014 and 3,696,817 describe a thresher containing two ormore rotating elements in a cascade, set in conventional fixed baskets.Smith in U.S. Pat. No. 3,706,314 describes a machine with two rotatingelements with radial teeth meshing with rotating elements consisting ofdiscs. None of these three machines is in common use.

Wochnowski in GB 1,077,410 and Johansson et al in U.S. Pat. No.3,229,698 describe threshers containing two or more mills with fixedbaskets contained within an air separation tower. In the first of these,the threshers are conventional in form. In the second, the thresher axisis mounted vertically rather than horizontally. Machines of the secondform are commercially used but they seem generally to be less efficientthan the conventional type and are used primarily where floor space isat a premium.

Phillips in GB 301,239 describes a machine for stripping tobacco leavesin which the lamina is separated from the stem by passing the leavesbetween pairs of counter-rotating brushes and rollers which rotate atdifferent speeds. The leaves are subjected to tensile forces whichseparate the-lamina from the stem.

Dahistrom et al in U.S. Pat. No. 2,150,493 and U.S. Pat. No. 2,152,791teach a device for disintegrating tobacco leaves which includes a pairof counter-rotating rollers for feeding tobacco leaves to a rotatingcylinder having a series of projecting teeth. However, all of theseparation of the lamina from the stem occurs away from the rollers atthe point where the teeth on the cylinder pass through a series ofintermeshing discs.

U.S. Pat. No. 2,789,564 (Hunter) and U.S. Pat. No. 4,805,643 (Tetaka)describe apparatus in which tobacco is delivered to a relatively largetoothed rotor via an opening located. above the rotor. Hunter employsfurther smaller toothed rotors which intermesh with the large rotor tothresh the tobacco leaves.

EP-A-0135048 relates to a system for use with a tobacco threshingmachine to control lamina size.

In the food and farming industries, threshing is traditionally used toobtain the seeds or fruits of the crop free from the bulk of the plantmaterial. This is normally done as a part of the harvesting.

Threshers used for legume crops use an axial flow threshing system asdescribed by Looker et al in GB 1,396,931 and 1,396,932. Here the cropmass is transferred into a large rotary drum constructed from meshpanels. Inside the drum are a number of beater elements.

According to Looker et al, these work by designing the crop mass flowpath such that several impacts occur. These impacts are sufficient tobreak open the legume pod and free the seeds contained inside.

The present invention relates to apparatus and a process for threshingtobacco which has significant advantages over the known techniques.

Accordingly, the present invention provides apparatus for threshingtobacco comprising rotatable stripping means and a feeder comprising apair of co-operating counter-rotatable elements for delivering tobaccoleaves to the stripping means, the feeder and the stripping means beingarranged such that, in use, the tobacco leaves experience shearingforces as they pass from between the counter-rotating elements to therotating stripping means which forces at least partially strip thelamina from the stem.

The stripping means preferably comprises radially extending arms. Thearms may be in the form of continuous plates or spaced teeth (which canbe straight, bent or curved) and may include parts which are capable ofcutting the tobacco leaves or are capable of puncturing the leaf andtearing through the leaf.

The elements also preferably comprise radially extending arms which alsomay be in the form of straight, curved or bent continuous plates orspaced teeth, optionally including parts which are capable of cuttingthe tobacco leaves. Alternatively, the feeder elements may comprise apair of rollers or may consist of a flexible membrane wrapped around aframework which allows variable quantities of leaf to be fed.

The arms may be flexible, inflexible or a mixture of fixed and flexiblecomponents.

The rotating elements and the stripping means can take the same form andmay be the same shape and size, differing only in their function whichis dictated by their position in the apparatus.

The stripping means and the feeder are arranged at a relatively closedistance from each other in order to subject the tobacco to the shearingforces which at least partially strip the lamina from the stem.Preferably, the distance between the outermost parts of the two elementsin the feeder is less than 150 mm and the distance between the outermostparts of each of these elements and the outermost parts of the strippingmeans is less than 100 mm. The shearing forces are experienced by thetobacco leaves as they are directed from the feeder to the rotatingstripping means. The tobacco leaves are preferably delivered to therotating stripping means substantially along a radius of the axis ofrotation of the stripping means. The lamina is stripped from the stem inthe region where the leaves pass from being moved under the influence ofthe feeder to being moved by the stripping means and, as the skilledperson will appreciate, the exact position of this region will varydepending upon the particular configuration of the apparatus and therate of rotation of its various rotating components. The shearing forcecan be considered, at least in certain circumstances, as arising fromthe action of the stripping means on one part of the leaf while thefeeder is holding another part of the leaf.

Preferably, where the arms of the stripping means and those of theelements consist of spaced teeth, the teeth are intermeshed. With suchan arrangement, the stripping of the lamina from the stem is effected,to some extent, by the interaction of the stripping means and thefeeder. When the teeth are intermeshed, the distance between the teethas they pass each other is typically less than 100 mm.

The apparatus preferably comprises a rotatable drum in which thestripping means and the feeder are arranged. The drum may be generallycylindrical and its walls may be solid. However, the drum preferablycomprises a screen (e.g., in its walls) which allows at least a part ofthe stripped tobacco to pass out of the drum. The screen can form all oronly part of the side walls of the drum.

The drum preferably incorporates internally protruding lugs fordirecting the tobacco leaves to the feeder elements. As the drumrotates, the lugs collect the tobacco leaves so that the leaves travelaround the inside of the drum until they fall (preferably solely underthe influence of gravity) into the feeder. The positioning of the feederwithin the drum and/or the speed of rotation of the drum are adjusted soas to ensure that a suitable amount of the tobacco leaves is deliveredto the feeder as the drum rotates.

To assist transfer through the drum, an additional fixed or rotatingelement, running down the drum parallel to the stripping means, may beused to slow down the tobacco leaving the stripping means. The tobaccothus slowed will then be able to fall under the influence of gravityonto a lower portion of the surface of the drum. Preferably thetangential speed of the additional rotating element is similar to thatof the drum (i.e., ±50% of the speed of the drum) and it is alsopreferred that the additional rotating element has a direction ofrotation opposite to that of the drum.

The stripping means and the elements preferably rotate about axes whichare parallel to the axis of rotation of the drum. Preferably, the axesof rotation are either substantially horizontal or are tilted at from 0°to 10° (more preferably from 2° to 6°) to the horizontal.

The stripping means and elements can converge as they approach the exitend of the drum to take account of reduced loading resulting from thesmall particles being sieved out of the main flow. This also allows theamount of threshing that takes place to increase towards the exit end ofthe drum. This convergence can be achieved by varying the diameter ofthe stripping means and/or the elements along their length. By adoptingthis method, an additional effect will be to increase the tangentialvelocity of the arm tips as the diameter increases and this will affectthe threshing characteristics.

The diameter of the drum will typically be in the range of from 300 to2500 mm, preferably from 900 to 1800 mm.

The diameters of the stripping means and the elements (as defined by theend of any radially protruding arms) are typically from 5 to 50% of thediameter of the drum.

The tangential velocity of the drum is preferably in the range of from0.04 to 0.5 m/s with the tangential velocity of the stripping means andthe elements being from 3 to 250 times (preferably 10 to 100 times)greater than that of the drum.

The rotating components of the apparatus (other than the drum) can berun at synchronous speeds to achieve true intermeshing of the strippingmeans and the rotating elements. Alternatively, the rotating componentscan be run such that they have different tangential velocities, thussetting up shear actions between the various rotating components. Thechoice of rotational speed and mode of operation depend upon the designof the rotating component and drum protrusions used and this in turndepends on the nature of the leaves being threshed and upon the requiredspecification of the threshed material.

The conveyer used for feeding the leaf material into the drum can beconstructed such that it will move the feed point of the leaves into thedrum. This can be used to control the amount of threshing that is doneon the leaves.

An open topped conveying device can also be incorporated to remove largepieces of free lamina from the rest of the leaf material. The action ofthe rotating components of the apparatus can be arranged to throw theleaf material through the air. The lighter stem free lamina slows downmore rapidly and lands on the conveyer while the heavier stem containingparts are thrown over the top of the conveyor and continue around thedrum for further threshing. This avoids unnecessary damage to the largerpieces of stem free lamina.

Directed air currents within the drum could be used to assist thisseparation.

In another embodiment, the present invention relates to a process forthreshing tobacco which comprises providing tobacco leaves to a feedercomprising a pair of co-operating counter-rotating elements and feedingthe leaves from the feeder to rotating stripping means such that theleaves experience shearing forces as they pass from between thecounter-rotating elements to the stripping means which forces at leastpartially strip the lamina from the stem. The process is convenientlycarried out using the apparatus of the invention.

The tobacco leaves which are threshed in the apparatus and process ofthe invention preferably have a moisture content of between 5 and 35% byweight and may have been conditioned before threshing.

The apparatus of the invention can comprise more than one (e.g., two)stripping means and the extra stripping means can be provided by one ormore additional rotating elements. These additional elements may carryout a degree of threshing and/or cutting of the tobacco leaves and theymay participate in the delivery of the partially threshed or unthreshedleaves to the feeder and/or the stripping means. The apparatus may alsocomprise additional rotating elements which act solely to direct leafmaterial into the stripping means.

The invention will now be described, by way of example only, withreference to the accompanying drawings, wherein:

FIG. 1 shows a cross-section through an apparatus according to oneembodiment of the invention;

FIG. 2 shows a cross-section through an apparatus according to anotherembodiment of the invention;

FIG. 3 shows a cross-section through an apparatus according to yetanother embodiment of the invention;

FIGS. 4 and 5 show perspective views of stripping means suitable for usein the apparatus of the invention;

FIG. 6 shows perspective views of three types of arm (or teeth) for usein the apparatus of the invention;

FIG. 7 shows a plan view of another arm for use in the stripping meansof the invention;

FIG. 8 is a side view of yet another stripping means suitable for use inthe apparatus of the invention; and

FIG. 9 is a flow diagram of a preferred process for threshing tobaccousing the invention.

Referring to FIG. 1, drum 6 rotates about a substantially horizontalaxis in an anti-clockwise direction. A series of lifting pins (or lugs)7 are attached to the inside surface of drum 6. The function of the pins7 is to move the tobacco leaves up the side of drum 6. The leaves arethen gathered by co-operating counter-rotating elements 1 and 2 whichform a feeder for the leaves. Elements 1, 2 comprise radially extendingarms 1 a, 2 a which run the full length of the drum. In the embodimentof the invention shown in FIG. 1, the arms 1 a, 2 a are as depicted inFIG. 5. The leaves are forced between elements 1,2 where some of theshearing or cutting of the leaves may take place. The leaves aredelivered from between elements 1,2 into stripping means 3 which hasarms 3 a. Stripping means 3 rotates in the same direction as the drum inthe Figure, i.e., anti-clockwise, although it may rotate in the oppositedirection. It is in the region of delivery of the leaves from elements1,2 to arms 3 a of stripping means 3 that the major part of thethreshing takes place as the leaves which are still partially held byelements 1,2 are torn by the shearing action of stripping means 3.Stripping means 3 may also act to cut the leaves to some extent. Theleaves threshed by stripping means 3, excluding any parts of thethreshed leaf which may have fallen t the bottom of drum 6, are gatheredbetween stripping means 3 and element 4 which is located further roundthe drum 6 (in the direction of its rotation) and are forced towardssecond stripping means 5 creating a second shearing zone within thedrum. The rotational direction of the rotating elements 1,2,4, thestripping means 3,5 and the drum 6 are indicated by the broken arrows.

Another embodiment of the invention is illustrated in FIG. 2. Here theleaves are carried up the side of the drum 10 on the lugs 11 by therotational movement of the drum 10 (anti-clockwise in the Figure). Theleaves are then drawn into the gap between the counter-rotating elements12 and 13 which run the length of the drum 10 and co-operate to act as afeeder which directs the leaves into stripping means 14. The mainfunction of stripping means 14 is to shear or rip the lamina free fromthe stems of the leaves. Stripping means 14 and elements 12,13 have bentarms 14 a,12 a,13 a and are of the general design shown in FIG. 4. Analternative design for the arms 14 a, 12 a, 13 a is illustrated in FIG.7.

In FIG. 3, drum 20 rotates anti-clockwise and, in use, lugs 21 transfertobacco leaves to counter-rotating elements 22 and 23. The differentlydesigned arms 22 a and 23 a of elements 22 and 23, respectively, feedthe leaves to stripping means 24 which has arms 24 a. Arms 24 aintermesh with arms 23 a. Fixed element 25 catches the threshed tobaccoas it leaves stripping means 24 and allows it to fall back down to theinside surface of drum 20.

Referring to FIG. 6, the arms 1 a, 2 a of the feeder and/or thestripping means may have a flat edge. Alternatively, as shown for arms23 a, 24 a, the outermost edge of the arms may be tapered to a point andmay taper from one end to a narrower other end along the drum directionin order to assist in the shearing of the leaves.

The process of the invention may be run continuously or as a batchprocess. For continuous operation, the tobacco leaves are fed into oneend of the drum, the leaves are threshed throughout the length of thedrum and the threshed leaves which have not already passed out of thedrum (e.g., through screens in its walls) exit the drum at the otherend. Passage of the leaves from one end of the drum to the other can beachieved by tilting the drum and, optionally, also the rotatingcomponents of the apparatus at an angle to the horizontal or byincluding a helical screw 15 in the apparatus, as shown in FIG. 8. Thehelical screw can constitute the stripping means and/or the rotatingelements of the apparatus.

Alternatively, the transfer of the leaf material down the drum can beeffected by arranging the lugs helically on the inside of the drum orthe transfer can be caused by air currents acting on the leaf material.

A preferred process for use with the invention involves recycling thethreshed tobacco as shown in the flow chart of FIG. 9. Where the drum isconstructed from screen sections, the whole device acts like a thresherand sieve in a single machine. In this way it can be incorporated intoan overall process as shown in FIG. 9 or it can be used as a part of aconventional process line. With reference to FIG. 9, tobacco leaves arefirst conditioned and are then threshed using the process and apparatusof the invention. Two fractions are obtained after threshing; (i)screened tobacco which is the smaller pieces of threshed tobacco whichhave passed through a screen (preferably in the side wall of a drum ofthe-apparatus) and (ii) large tobacco which is the larger pieces oftobacco retained in the apparatus. The screened tobacco, made up of thesmaller particles of the threshed leaves, is subjected to conventionalair separation and the “heavies” and “lights” fractions go on to dryingor further processing in the conventional manner. The large tobaccopieces are also subjected to air separation and the lights sent on todrying or further processing. However, the heavies are directed back tothe threshing stage of the process to separate more of the lamina fromthe stem. This type of process is described in detail in our relatedEuropean patent application no. 96309188.9 which was filed on the sameday as this application and has the title “Method And Apparatus ForProcessing Tobacco”.

As mentioned above, the surface of the drum used in the apparatus of theinvention, which is preferably cylindrical, can be either solid or maycomprise screens.

Where the drum surface is solid, the whole mass of leaves travels thefull length of the drum passing through the stripping means.

Where the drum surface consists of screens, smaller threshed leafparticles pass out of the drum without having to traverse the fulllength of the drum. This means that the smaller particles will avoidfurther unnecessary impacts and will suffer less damage as a result oncethey have been threshed. Larger particles unable to pass through thescreens travel the full length of the drum and exit from the end of thedrum. The screens have a mesh size suited to the requirements of thedesired final product.

Where the process requires an improved screening efficiency, the lengthof the-elements and the stripping means can be less than the full lengthof the drum. This allows extra screening to be carried out before, afteror before and after the threshing zone by suitable location of theelements and stripping means within the drum. Stem free lamina cantherefore be removed prior to threshing, eliminating unnecessary damageto it and the amount of clean stem being removed through the screenafter threshing can be increased.

The apparatus, where the drum surface consists of screens, can be usedto thresh filler for use in cigars. The filler which is small enough tobe used in the manufacture of cigar rods will pass through the screen ofthe drum. The oversize material will be recycled back into the feed endof the drum, as shown in FIG. 8, and rethreshed until it is small enoughto pass through the screens.

The following non-limiting examples further illustrate the invention.

EXAMPLE 1

A sample of tangled tobacco leaf, with a moisture content of 27.8% byweight, was threshed for 80 seconds in a batch in a machine configuredas shown in FIG. 2. The tangential velocity of the drum was 0.1 8 m/s.The stripping elements, with arms as shown in FIG. 7, were rotating witha tangential velocity of 1.5 m/s and the feeding elements with arms asshown in FIG. 4, were rotating with a tangential velocity of 1.0 m/s.

The machine had a drum of diameter 1524 mm, the diameter of thestripping means (including its arms) was 455 mm and that of each of theelements of the feeder (including arms) was 305 mm. The gap between thearms of the elements of the feeder was about 100 mm and the edge of thearms of the stripping means was located about 20 mm away from the armsof the feeder elements. 60.3% of the total lamina input was removed aslamina free from stem, and the lamina contained 83% greater than 12.7 mm(½″) and 6% smaller than 6.35 mm (¼″).

EXAMPLE 2

A sample of tangled tobacco leaf, with a moisture content of 27.2% byweight, was threshed for 30 seconds in a batch in the machine describedin Example 1. The tangential velocity of the drum was 0.17 m/s. Thestripping elements, with arms as shown in FIG. 7, were rotating with atangential velocity of 3.0 m/s and the feeding elements with arms asshown in FIG. 4, were rotating with a tangential velocity of 2.0 m/s.73.6% of the total lamina input was removed as lamina free from stem,and the lamina contained 88.2% greater than 12.7 mm (½″) and 3.3%smaller than 6.35 mm (¼″).

EXAMPLE 3

A sample of tangled tobacco leaf, with a moisture content of 20.7% byweight, was threshed for 20 seconds in a batch in the machine describedin Example 1. The tangential velocity of the drum was 0.17 m/s. Thestripping elements, with arms as shown in FIG. 4, were rotating with atangential velocity of 2.95 m/s and the feeding elements with arms asshown in FIG. 1, were rotating with a tangential velocity of 1.97 m/s.75.9% of the total lamina input was removed as lamina free from stem,and the lamina contained 87.5% greater than 12.7 mm (½″) and 3.9%smaller than 6.35 mm (¼″).

EXAMPLE 4

A sample of tangled tobacco leaf, with a moisture content of 25.2% byweight, was threshed for 20 seconds in a batch in the machine describedin Example 1. The tangential velocity of the drum was 0.18 m/s. Thestripping elements, with arms as shown in FIG. 7, were rotating with atangential velocity of 3.94 m/s and the feeding elements with arms asshown in FIG. 4, were rotating with a tangential velocity of 1.53 m/s.64.4% of the total lamina input was removed as lamina free from stem,and the lamina contained 88.5% greater than 12.7 mm (½″) and 3.4%smaller than 6.35 mm (¼″).

The advantages of the apparatus of the invention over conventionalthreshers can be summarised as follows:

(a) Larger pieces of free lamina can be produced as they do not have tobe reduced in size to pass through the thresher basket.

(b) Less dust is produced from the leaves as no grinding of the leavesoccurs between the thresher rotor and the surfaces of the machine.

(c) The total plant to complete the threshing of leaves is reduced as upto 90% of the lamina can be freed from the stem in a single pass,compared to less than 70% for a conventional thresher.

What is claimed is:
 1. Apparatus for threshing tobacco comprisingrotatable stripping means and a feeder comprising a pair of co-operatingcounter-rotatable elements for-delivering tobacco leaves to thestripping means, the feeder and the stripping means being arranged suchthat, in use, the tobacco leaves experience shearing forces as they passfrom between the counter-rotating elements to the rotating strippingmeans which forces at least partially strip the lamina from the stem. 2.Apparatus as claimed in claim 1, wherein the stripping means comprisesradially extending arms.
 3. Apparatus as claimed in claim 1, wherein theelements comprise radially extending arms.
 4. Apparatus as claimed inclaim 1, wherein the stripping means and the feeder are arranged withina rotatable drum.
 5. Apparatus as claimed in claim 4, wherein the drumcomprises a screen which allows at least a part of the stripped laminato pass out of the drum.
 6. Apparatus as claimed in claim 4, wherein thedrum comprises internally protruding lugs for directing the tobaccoleaves to the feeder.
 7. Apparatus as claimed in claim 4, wherein thestripping means and the elements rotate about axes which are parallel tothe axis of rotation of the drum.
 8. Apparatus as claimed in claim 7,wherein the axes of rotation are substantially horizontal.
 9. Apparatusas claimed in claim 1, which comprises two rotatable stripping means.10. Apparatus as claimed in claim 1, which further comprises means forreducing the velocity of the tobacco leaves after they exit from thestripping means.
 11. Apparatus as claimed in claim 10, wherein saidmeans for reducing the velocity of the tobacco leaves is either fixed orrotates and has its axis aligned substantially parallel to the axis ofrotation of the stripping means.
 12. Apparatus as claimed in claim 2,wherein the arms are in the form of plates or teeth.
 13. Apparatus asclaimed in claim 12, wherein the arms of the stripping means and thearms of at least one of the elements are intermeshed.
 14. Apparatus asclaimed in claim 2, wherein the arms are flexible.